In a conventional exhaust gas recirculation device, the device having a structure in which an exhaust gas recirculation passage (hereinafter, referred to as EGR pipe) is connected to an intake-and-exhaust passage of an engine and the EGR pipe is equipped with an EGR valve alone has the disadvantage that deposits such as suspended matters contained in the circulating exhaust gas deposit on the inner peripheral surface of the EGR pipe and the valve-rod sliding area of the EGR valve and further, there arise pressure losses and malfunctions of the EGR valve, caused by the deposit deposition thereon. For this reason, an exhaust gas recirculation device has been already known, in which for the purpose of only removing the deposits, the downstream side of the EGR pipe and the exhaust passage are connected or associated by a bypass passage, the connection between the bypass passage and the EGR pipe is equipped with a switching valve, and this switching valve and the EGR valve are controlled in switching operation to bring, during a non-EGR operation, the high-temperature exhaust gas to burn and remove the deposits adhered to the inner wall during an EGR operation (see, for example, JP-A-10-299579, pp. 4-5, and FIG. 1). Also, an exhaust gas recirculation device has been already known, which is equipped with an EGR cooler placed in the exhaust gas recirculation passage in order to cool the high-temperature exhaust gas and circulate the gas to the engine intake side (see, for example, JP-A-2003-097361, Abstract, and Selected Drawing).
Patent Reference 1: JP-A-10-299579
Patent Reference 2: JP-A-2003-097361
The conventional exhaust gas recirculation devices are configured as mentioned above. In the case of JP-A-10-299579, the high-temperature exhaust gas is schemed to flow from the bypass passage into the EGR pipe through the exhaust passage during the non-EGR operation to thereby burn and remove the deposits adhered to the inner wall of the EGR pipe, and the EGR pipe is not provided with an EGR cooler. Because of this, an effect other than the deposit removal performed by the high-temperature exhaust gas, that is, the effect of cooling the high-temperature exhaust gas and circulating the cooled gas to the engine-intake side cannot be achieved. Furthermore, there is a problem that the EGR valve and the switching valve are susceptible to heat due to the high-temperature exhaust gas. Moreover, in the case of JP-A-2003-097361, the device is provided with an EGR cooler, and is arranged to open and close the bypass valve only when the warming up is accelerated and stabilized. Consequently, there is a problem that the high-temperature exhaust gas containing steam, unburned gas and oil passes through the EGR cooler, and thereby, condensed deposits adhere to the EGR valve, thus causing the EGR valve to stick. It is seemingly effective to incorporate the EGR cooler described by JP-A-2003-097361 into the EGR pipe described by JP-A-10-299579; in this case, because the exhaust gas cooled by the EGR cooler is, however, circulated to the exhaust passage through the exhaust passage, the exhaust gas cooled by the EGR cooler cannot be circulated to the intake side of the engine. As a result, there arises a serious problem that the incorporation is impractical.
The present invention has been accomplished to solve the above-mentioned problems. An object of the present invention is to provide an exhaust gas recirculation device that can cool the high-temperature exhaust gas and circulate the cooled gas to the engine-intake side, and, in spite of the fact, that can remove the deposits adhered to the exhaust gas recirculation passage and valve by the high-temperature exhaust gas, the device having improved reliability and durability because of the reduction of the pressure loss caused by the deposits.